OBJECTIVE: To describe the use of antibiotics in Peruvian children under 1 year in a setting where they are available without a prescription. METHODS: Data were analyzed from a cohort study between September 2006 and December 2007 of 1 023 children < 2 months old in periurban Lima, Peru, followed until they were 1 year old. RESULTS: Seven hundred seventy of 1 023 (75.3%) children took 2 085 courses of antibiotics. There were two courses per child per year (range 0-12). Higher rates of antibiotic use were found in children 3-6 months old (37.2%). Antibiotics were given to children for 8.2% of common colds, 58.6% of all pharyngitis, 66.0% of bronchitis, 40.7% of diarrheas, 22.8% of dermatitis, and 12.0% of bronchial obstructions. A physician's prescription was the most common reason for antibiotic use (90.8%). Medication use without a prescription was found in 6.9% of children, and in 63.9% of them it was preceded by a physician's prescription. CONCLUSIONS: Infants are often exposed to antibiotics in this setting. Overuse of antibiotics is common for diagnoses such as pharyngitis, bronchitis, bronchial obstruction, and diarrhea but is typically inappropriate (83.1% of courses) based on the most common etiologies for this age group. Interventions to improve the use of antibiotics should focus on physicians, since a physician's prescription was the most common reason for antibiotic use.